The known cholesterol lowering properties of soy and derivatives thereof are related with the content in isoflavones (Kirk et al., 1998) and in proteins (Anderson et. al, 1995).
Soy proteins mainly consist of glycinins (11S fraction) and β-conglycinins (7S fraction), the latter consisting of three subunits, named α, α′ and β (Thanh and Shibasaki, 1976). Studies carried out on soy proteins have established that the 7S fraction (Lovati et. al, 1992, 1996), particularly the α′ subunit (Manzoni et. al, 1998) is capable of activating LDL receptor and is therefore the main responsible for the reduction of cholesterol plasma levels. In fact, treatment of an hepatic cell line with 7S globulin induces extensive degradation of the α and α′ subunits and stimulation of LDL receptor activity, whereas β subunits are not degraded and the receptor is not activated. Moreover, soy mutants in which 7S fraction lacks α′ subunit are not able to modify the receptor activity, even at high concentrations.
As a consequence of these experimental observations, methods are needed to obtain β-conglycinin in the pure form, as well as recovering and purifying the α′ subunit, from which specific amino acidic sequences could subsequently be obtained by enzymatic treatment, without making use of peptide synthesis.
The process suggested by Than et al. (1975 and 1976) and subsequently modified by O'Keefe et al. (1991) allows to separate glycinins and β-conglycinins based on their different solubilities at different pH; however, cross-contamination is still high and gel filtration or affinity chromatography are required, which are costly and difficult to carry out on an industrial scale. Also the modification suggested by Nagano et al. (1992), although allowing to increase the fractions purity, is still an expensive method which can be used only on laboratory scale.
Recently, Wu et al. (1999) have described a method for separating glycinins and conglycinins on a pilot-plant scale. Glycinins are precipitated by two subsequent aqueous extractions at pH 8.5, followed by treatment of the supernatant with a 0.98 g/L bisulfite solution, while conglycinins are precipitated by adding 0.25 M NaCl to the mother liquors from the glycinins precipitation, then adjusting pH to 4.8. The process allows to treat high amounts of starting material and also provides high yields in protein, but the fractions purity is still unsatisfactory; β-conglycinin, in particular, undergoes degradation, apparently during diafiltration with water, which is a treatment necessary to reduce the bisulfite ions excess and to remove salts.
The above cited methods not only do not yield pure β-conglycinin, but above all do not envisage separation and purification of the α′ subunit.
According to the invention, a solid fraction enriched in β-conglycinin is prepared by extracting a defattedground soy in an aqueous medium according to conventional procedures and subsequently precipitating the supernatant with aqueous ethanol; the resulting fraction is then purified by Metal Affinity Chromatography (MAC) in denaturant conditions to yield the pure α′ subunit, which is subjected to enzymatic treatment with chymotrypsin to obtain the amino-terminal region which has apparently the highest LDL receptor-activating activity.